An ion exchange route to produce WO3 nanobars as Pt electrocatalyst promoter for oxygen reduction reaction
Highlights
► WO3 nanobars (3 nm × 10 nm) is synthesized through ion exchange route. ► The long-chain of the ion exchange resin results in the bar shape of WO3. ► The shape of WO3 varies with the concentrations of WO3 precursor. ► WO3 nanobars give highly promotion effect on Pt/C electrocatalyst. ► Pt/WO3 has higher electrochemical stability than Pt/C.
Introduction
Low temperature fuel cell technology has been considered as one of the important ways to decrease the energy crisis of the world for its high-energy conversion efficiency, so it has great and potential economic value [1], [2], [3], [4], [5]. The electrocatalyst is the critical part of the low temperature fuel cells. Owing to the sophisticated synthesis technologies, relatively good electrochemical activity and stability, noble metals such as Pt and Pd have been remaining the main active ingredients of the electrocatalysts [2], [6], [7], [8], [9]. However, the high cost of the noble metals restricted the popular application of the low temperature fuel cells, especially for the cathode side which need more amount of noble metals to promote the kinetics of the oxygen reduction reaction (ORR) [10], [11], [12]. On the other hand, the carbon support corrosion remains the major challenges for high potential on fuel cell ORR electrocatalysts [13]. Therefore, many efforts such as structure design, alloy synthesis and crystal parameter alteration on the noble metal have been made to obtain improved electrocatalytic activity, stability and use ratio [2], [8], [14], [15], [16]. On the above basis, graphitization or composite synthesis on the carbon support to achieve more stable catalyst structure and performance was also exposited [3], [7], [17], [18].
Metal oxides have aroused much attention and been studied in various fields including light-emitting diodes [19], [20], coating materials [21], photocatalysis [22], [23], [24], flame retardants [25], chemi-sensors [26], [27], solar cells [28], [29] and catalytic reaction [30], [31]. Due to their promotion effect on noble metal electrocatalyst, metal oxides were also investigated with interesting as electrocatalyst promoter for fuel cells, which have been approved to be able to improve the overall catalytic activity to a large degree [32], [33], [34]. Specially, it has been reported that tungsten oxides have excellent CO tolerance and higher catalytic activity when being loaded with Pt nanoparticles as electrocatalysts [35], [36], [37], [38], [39], [40], [41]. According to Savadogo's report, the electrocatalytic activity of Pt supporting on WO3 for ORR in acidic media was twice as high as that of Pt supporting on carbon [42].
There have been many methods for the synthesis of tungsten oxides with various shapes including nanowires [39], [40], nanofilms [43], nanoclusters [44], microfibers [45], etc. It is believed that the big particles of promoter catalyst are difficult to exert the promotion effect on noble metal electrocatalysts. The reason is that they have too heavy density and low specific surface area to perform the promotion effect efficiently and disperse the noble metal particles uniformly [46], [47]. So it is of great significance to develop nanosized metal oxides with a controllable size down to 10 nm or less.
Here, we synthesized WO3 nanobars with the length of 10–50 nm and the width of 3–6 nm that were uniformly dispersed on carbonized resin (C-WO3) through ionic exchange route [3], [7], [18]. The WO3 nanobars synthesized here were used as Pt electrocatalyst promoter for ORR in acidic media and showed excellent electrochemical stability as well as remarkable catalytic promotion effect.
Section snippets
Experimental
Typically, the D201 × 1 cinnamic strong alkali anion exchange resin (10 g, Hebi Power Resin Factory, China) was impregnated in 100 ml of AMT (ammonium metatungstate, (NH4)6W7O24·6H2O, A.R., Tianjin Jinke Fine Chemicals Co., China) solution with the W atom concentrations of 0.50, 0.05 and 0.005 mol L−1 for 5 h, respectively (W atom concentration equals 1/7 AMT concentration, for one AMT molecule having seven W atoms). The exchanged resin was washed with deionized water and dried at 80 °C overnight. The
Results and discussion
Fig. 1a and b show the XRD patterns of C-WO3(0.50), C-WO3(0.05) and C-WO3(0.005). All the patterns match the characteristics of WO3 crystal (PDF#20-1324) by comparing JCPDS cards. The peak intensities of the WO3 weakened with the decreasing concentration of AMT, which related to the particle size.
Fig. 2 shows the TEM images of the WO3 nanoparticles on carbonized resin, which were prepared at different concentrations of AMT. As shown in Fig. 2a, the C-WO3(0.50) prepared at higher concentration
Conclusions
Tungsten oxide nanobars with the length of 10–50 nm and the width of 3–6 nm supported on carbonized resin (C-WO3) were synthesized through an ionic exchange route to locally anchor the metatungstate ions. The produced WO3 crystals could have dot, bar or bulk shapes, which can be easily controlled by adjusting the concentration of the metatungstate ions. Pt nanoparticles were supported on the C-WO3 composites (Pt/C-WO3) and used as electrocatalyst for ORR. The results showed that WO3 with moderate
Acknowledgements
This work was financially supported by Research Foundation for Talented Scholars of Jiangsu University (11JDG142), Science and Technology Support Program of Jiangsu (BE2010144), Natural Science Foundation of Jiangsu (BK2010166) and Doctoral Fund from National Ministry of Education (20093227110009), China. Dr. X.M. Lü is gratefully acknowledge the financial support by National Natural Science Foundation of China (no. 21003065).
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